Process for heat treating nickelchrome-tungsten alloy steels



A. W. WHEELER May 23, 1944.

PROCESS FORHEAT TREATING NICKEL-CHROME-TUNGSTEN ALLOY STEELS `Filed4 April l, 1942 SSS Inventor:

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In steam turbines and the like the turbine casing is held together by very large steel bolts of which the notched impact strength is an important characteristic. While the Charpy impact for the bolting material commonly in use varies from about 23.5 to about 33 foot pounds the same material with a 60 V notch requires only to about? foot pounds to produce a failure.

In carrying out my invention I employ as the turbine bolting material a low nickel-chrome steel which contains a small percentage of tungsten and generally less than 1% of nickel. The steel may contain about .12% to .15% carbon, about .5% to 2% nickel, about 12 to 14% chro.- mium and about 2.5 to 3.15% tungsten with the remainder substantially all iron. A steel which consists of about .15% carbon. .53% nickel, 12.84% chromium, 2.64% tungsten with fractional percentages of manganese and silicon gives satisfactory results.

My improved heat treatment comprises heating the alloy for several hours, for example B hours, at about `1100 C. :25 C., quenching in oil, reheating to about 600 to 700 C. for a few hours, for example about 4 hours, for 3" diameter material and then air cooling. An alloy of 30' the above noted composition when given this heat treatment had a Charpy impact resistance of 24 to 33 foot pounds but when provided with a 60 V notch the same material required an impact of 11.7 to 27 foot pounds to -producea failure. The best results were obtained by heating the steel at about 11.00 C. for about 8 hours, air cooling or quenching in oil or water, and then reheating at 650 C. for 4 hours followed by air cooling.

In the accompanying drawing the results of tests at various elevated and intermediate temperatures are set forth. These tests show that for material having a 60 V notch the heat treatment at about 1100 to about 1200 C. followed by a quench and reheat at 600 to 700 C. produces unusual results. These tests also indicate clearly that for ordinary commercial heat treatments as set forth in tests i to i5 inclusive, the above noted alloy is very sensitive to sharp notches such as those present in screw threaded bolts but that there is a narrow `range of heat treatment in which the sharp notch sensitivity is substantially eliminated. Material which has been heat treated in accordance with my process not only eliminates the sharp notch sensitivity but also has improved resistance to rupture and improved creep strength .at high temperatures. Moreover, fractures produced in the steel are oi' a ductile character.

` What I claim as new and desire to secure by Letters Patent of the United States, is:

i. The method for heat treating chrome steel containing not more than about .15% carbon, about 2.6% tungsten. about .5% nickel and about 13% chromium which comprises heating the steel at about 1100 to 1200 C. for about 3 hours, quenching the steel and then reheating it at about 600 to700" C. for about 4 hours.

2. The method for heat treating chrome steel containing about 0.12 to 0.15% carbon, about .5 to 2% nickel, about 12 to 14% chromium, about 2.5 to 3.15% tungsten with the remainder iron which comprises heating the steel at a temperature oi about 1100 C. for about 8 hours,- quenching the steel and thereafter reheating it at about 600 to '700C. for a few hours. r

3. The method for improving the notched impact strength of steel bolting material cbntaining about .12% to .15% carbon, about 2.6 tungsten, about .5% nickel, about 13% chromium with the remainder substantially all ironwhichl comprises heating the steel at about 1100 to 1200 C. for about 8 hours. quenching the steel and thereafter reheatlng it at about 600 to 700 C. for a few hours.

4. The method for heat treating steel contain- 40 ing not more than about 0.15% carbon, about 0.5 to 2% nickel, about 2.5 to 3.15% tungsten with the remainder substantially all iron which comprises heatingthe steel at about 1100 to 1200 C. for several hours, quenching it from that temperance and thereafter :cheating the steel at about 600 to 10. for a few hours. 

